This invention relates to a method of purifying crude hexafluoroacetone which is obtained by reaction between hexachloroacetone and hydrogen fluoride and contains chlorofluoroacetones as well as hydrogen halides.
Hexafluoroacetone is useful as an intermediate of fluorine-containing polymers and some medicines and agricultural chemicals and also as a solvent for various polymeric materials.
As is well known, hexafluoroacetone can be prepared with fairly good yield by contact reaction between hexachloroacetone and anhydrous hydrogen fluoride in the presence of a suitable catalyst, for example a combination of chromium trifluoride and dichromium trioxide as disclosed in Japanese Patent Application Publication No. 40(1965)-27173. However, it is inevitable that the product of this reaction contains hydrogen chloride and/or hydrogen fluoride, and it is difficult to completely remove such hydrogen halides from the reaction product to thereby isolate hexafluoroacetone because each hydrogen halide readily combines with hexafluoroacetone to form a complex compound.
For example, Japanese Patent Application Publication No. 46(1971)-6761 proposes a complicated method of purifying crude hexafluoroacetone containing hydrogen halides. This method has the steps of preliminarily converting hydrogen fluoride in the crude material into hydrogen chloride, hydrating the crude material, removing a major portion of hydrogen chloride by heating, dehydrating the remaining hexafluoroacetone hydrate by using sulfuric acid, and finally making the dehydrated and accordingly gasified hexafluoroacetone contact with a neutralizing agent, which is a polybasic salt of either an alkali metal or an alkaline earth metal, in order to remove a trace amount of acidic impurity still present in the gaseous hexafluoroacetone. Besides the complicatedness of the procedure, it is a shortcoming of this method that removal of hydrogen fluoride which is usually contained in the crude material can be achieved only to a limited extent.
Other than the aforementioned hydrogen halides, some chlorofluoroacetones such as chloropentafluoroacetone, dichlorotetrafluoroacetone and trichlorotrifluoroacetone are usually present in a crude hexafluoroacetone obtained by reaction between hexachloroacetone and hydrogen fluoride. Unfortunately, it is quite difficult to separate these chlorofluoroacetones from hexafluoroacetone by distillation because these chlorofluoroacetones readily combine with the hydrogen halides similarly to hexafluoroacetone to form complex compounds. Moreover, some of these chlorofluoroacetones, e.g. chloropentafluoroacetone and dichlorotetrafluoroacetones, readily react with water analogously to hexafluoroacetone to form hydrates of various degrees of hydration, and the boiling points of stable hydrates of such chlorofluoroacetones and hexafluoroacetone are all within the narrow range from 105.degree. to 106.degree. so that it is impossible to separate hydrated hexafluoroacetone from the hydrates of the chlorofluoroacetone by distillation.
However, these chlorofluoroacetones (particularly chloropentafluoroacetone and dichlorotetrafluoroacetones) are highly toxic and therefore must completely be removed from hexafluoroacetone for use as an industrial material. Preferentially to physical separation methods such as distillation, there has been an eager demand for a good chemical process of completely decomposing such chlorofluoroacetones without decomposing hexafluoroacetone, but the demand has not yet been satisfied. A primary reason for the unsuccess is that both hexafluoroacetone and the chlorofluoroacetones are very high in reactivity. That is, hexafluoroacetone readily combines with an acidic compound such as a halogen-containing acid to form a complex compound and also readily reacts with an alkaline compound such as potassium hydroxide or sodium hydroxide to decompose into trifluromethane and a metal salt of trifluoroacetic acid. Besides, it readily reacts with various organic compounds, e.g. amines and alcohols. These chemical properties of hexafluoroacetone are almost similarly possesed by chlorofluoroacetones as homologeous compounds. Therefore, it has been accepted as quite difficult to separate chlorofluoroacetones from hexafluoroacetone by utilizing certain differences in chemical reactivities.